The use of radioactive sources for nondestructive testing of vital components of industrial systems poses radiation hazards for radiographic technicians and personnel in the area where the testing is being done. To avoid unacceptable exposure to radiation by radiographic technicians, personal dosimeters, ion chamber or Geiger Mueller tube based survey meters are used to indicate the gamma dose rates at the position of the technician. In addition to providing an analog display of the dose rate when inserted into the field, the survey meters, which can employ ion chamber or Geiger Mueller tubes as gamma ray sensing elements, can be equipped with audible alarms. There are also personal dosimeters that are worn by technicians that provide direct and indirect means of monitoring instantaneous and integrated radiation dose exposures. Some of these devices, which are attached to the clothing of technicians, can provide an audible alarm whenever the gamma field at the technicians' location exceeds a prescribed dose rate.
Radiographic camera systems consist of storage containers that contain depleted uranium shielding materials wherein the source resides during nonuse. Positive mechanical control of the source as it is moved in and out of the shield and through the source guide tube is achieved via a mechanical linkage between the source and a hand cranked mechanism. These systems do not contain any active method based on the detection of radiation to verify whether or not the source was parked in the shield after use. That is, even though the driving cable is retracted there is no existing method od determining whether or not the source was returned to the shield by the mechanically driven control units other than the active survey of the area with a survey meter. To determine whether or not the source was mechanically withdrawn, the technician must approach the radiographic camera-source guide tube area with the survey meter. If the source was not returned to the shield, the technician risks exposing him or herself while attempting to determine if a hazard exists. These sources, which typically have activities as large as 100 Curies can pose serious health risk to users. Because of the risk, all industrial radiographers and assistants must wear integrating radiation dose measuring devices and alarming dose ratemeters.
Given the potential for serious radiation exposure to a practicing radiographic technologist and to pedestrian traffic at the facility where the irradiation is being performed, it was desired to provide an invention that would automatically provide a visible or audible alarm when a source is driven out of its shield. A warning indicator that would remain activated until the source is retracted back into the shield.
A second objective of the present invention is to provide a source position monitor with a rugged large area self-powered detector that would be sensitive to photons with different energies. A source monitor that provides a signal to activate a local source/out indicator at the radiographic camera and a remote source out indicator. Such a remote indicator may be located at a fixed highly visible position or worn by the radiographic technologist.
A further objective of this invention was to provide a source position monitor that would not require an external power source to power the photon detector in the monitor.
These and other objectives and advantages of the present invention will become readily apparent from the following accompanying drawings.